The patient with complex congenital heart disease, severe pulmonary outflow obstruction, and visceral heterotaxia, may have 'silent' obstruction of the pulmonary venous return. Severe reduction of pulmonary blood flow secondary to pulmonary stenosis or atresia in such patients may prevent the usual radiographic appearance of pulmonary oedema. If such obstructed anomalous pulmonary venous connections are not diagnosed before operation, construction of a systemic to pulmonary artery anastomosis will unmask the obstruction, usually resulting in pulmonary oedema and death. We have recently 'challenged' a neonate with dextrocardia, vesceral heterotaxia, presumed asplenia, and complex congenital heart disease including pulmonary atresia, with an infusion of prostaglandin E1 to increase pulmonary blood flow via his ductus arteriosus. This resulted in severe pulmonary oedema which partially resolved after the infusion was discontinued. This was interpreted as consistent with obstructed total anomalous pulmonary venous return. This was confirmed at necropsy. Thus, the infusion of prostaglandin E1 before operation in the patient with asplenia or similar cardiac disease may be of aid in unmasking 'silent' obstructions of the pulmonary veins, and is of obvious value in the preoperative assessment of such patients.